Structure of the Field Cages

The field cages are designed to be low mass and "thin" for particles passing through them yet they must also be gas tight and strong enough to be self supporting.  So the field cages were built using two sheets of metal coated Kapton separated by a honeycomb of Nomex.  The whole assembly was rolled into a tube and the sheets of Kapton were epoxied to the honeycomb to form a strong sandwich of material.

The Kapton in the outer field cage (OFC) is laminated with a 35 micron  layer of copper and the metal layer is etched into stripes so that, after rolling the tube, the stripes become rings around the tube.

Width of the rings  10.0  mm
Width of the gap between rings    1.5  mm
Pitch of the stripe pattern (ring to ring)  11.5  mm
Number of rings  182
The inner field cage (IFC) is similar to the outer field cage but the Kapton is laminated with a thinner layer of Aluminum (9 micron) and the Nomex layer is thicker (1.27 cm). 

The two sides of the tube are connected by metal pins.  However, the stripes on one side of the tube are not aligned with the stripes on the other side.  They overlap, instead, with the stripes on one side centered over the gap on the opposite side.  This allows the metal layers to become part of the mechanical structure of the tube which increases its strength.   This also puts metal opposite the gap in the rings facing the TPC volume.  The image charges developed on the opposite side of the gap are supposed to reduce the field distortions that might be caused by charge build up on the exposed Kapton in the gap facing the TPC.
Structure Material cm g/cm**3 Rad Length g/cm**2  g/cm**2 Rad Length %
Insulating gas N2 30.00 0.001 37.99 0.038 0.10
TPC IFC Aluminum 0.004 2.700 24.01 0.010 0.04
TPC IFC Kapton 0.015 1.420 40.30 0.022 0.05
TPC IFC Nomex 1.27 0.064 40  0.081 0.20
TPC IFC Adhesive* 0.08 1.20 40 0.091 0.23
IFC Total (w/gas) 0.62
 Source:  Russ Wells           *Adhesive is only an estimate.
Structure Material cm g/cm**3 Rad Length g/cm**2 g/cm**2 Rad Length %
TPC gas P10 150.00 1.56e-03 20.04 0.234 1.17
TPC OFC Copper 0.013 8.96 12.86 0.116 0.91
TPC OFC Kapton 0.015 1.420 40.30 0.022 0.05
TPC OFC Nomex 0.953 0.064 40  0.061 0.15
TPC OFC Adhesive* 0.05 1.20 40 0.060 0.15
OFC Total (w/gas) 2.43
 Source:  Russ Wells           *Adhesive is only an estimate.

The outer field cage is nested inside an aluminum gas containment vessel which presents additional radiation lengths of material to particles passing out of the TPC in the radial direction.   It is separated from the OFC by 5.7 centimeters of nitrogen gas.  The gas vessel also has aluminum brackets glued onto its outer circumference to support the CTB trays and to temperature stabilize the TPC.

Structure Material cm g/cm**3 Rad Length g/cm**2 g/cm**2 Rad Length %
Insulating gas N2 5.70 0.001 37.99 0.007 0.02
Gas Vessel Aluminum 0.40 2.590 24.01 1.036 4.31
Gas Vessel Honeycomb 0.60 0.037 24.01 0.022 0.09
Gas Vessel Al Brackets 0.635 2.590 24.01 1.645 6.85
Gas Vessel Adhesive* 0.157 1.20 40 0.189 0.47
Gas Vessel Total 11.74
 Source:  Russ Wells           *Adhesive is only an estimate.

Finally, you might also want to consider the beam pipe, the SVT, and the CTB trays if you are interested in the total radiation length budget between the vertex and some part of the detector.  The beam pipe is approximately 1 mm of beryllium which is equivalent to 0.28% radiation lengths.  And the SVT is roughly 1.5% radiation lengths without the fourth layer.

Page created by Jim Thomas, send comments to
Last modified on March 31st, 1998